WASHINGTON (AP) - Scientists are still unable to determine if there is a link between a mercury-containing preservative used in some vaccines and disorders in children, the National Academy of Sciences said.

The ingredient, thimerosal, has been removed from most vaccines and the academy said that, despite the lack of proof that it is a hazard, prudence dictates that steps be taken to further reduce its use.

Safe Minds, an advocacy group working to reduce children's exposure to mercury, welcomed the report but contended it didn't go far enough.

Safe Minds president Sallie Bernard said the group is pleased the report acknowledges the possibility of the preservative being linked to health problems. But she said the group is renewing its call for removal of all childhood vaccines containing thimerosal.

The connection between exposure to high levels of mercury and problems with the nervous system has long been known.

While thimerosal contains a different form of mercury than the one that has been implicated in nervous disorders, critics have complained that it also may pose a hazard.

More than 35 law firms have formed a coalition to try to force the pharmaceutical industry to pay for studies to determine whether trace amounts of mercury in vaccines caused autism and learning disabilities in young children.

The coalition, led by an Oregon firm, is combining individual complaints filed since last May to create a national class-action lawsuit.

The Mercury Vaccine Alliance represents families in at least 25 states who claim that millions of children were harmed by exposure to mercury contained in thimerosal.

Thimerosal was used for many years to prevent bacterial contamination of vaccines. Currently, however, few vaccines given to children in the United States contain the product.

It was never used in vaccines against measles, mumps, rubella, chicken pox and polio. However, until recently, some other vaccines on the recommended childhood immunization list used it.

They are now manufactured without thimerosal, but a small number of doses for hepatitis B; diphtheria, tetanus and pertussis; and influenza type B with thimerosal may still be on clinic shelves, according to the report by the Academy's Institute of Medicine.

"Most children in the United States being immunized today and in the future are unlikely to receive a vaccine that contains thimerosal," said committee chair Marie McCormick, professor of maternal and child health at Harvard School of Public Health in Boston.

"In those few cases where only supplies containing the preservative are available, the vaccines should be administered rather than foregoing immunization," she said. "While the health effects of thimerosal are uncertain, we know for sure that these vaccines protect against real, proven threats to unvaccinated infants, children, and pregnant women."

The institute said it conducted an extensive analysis of studies assessing whether thimerosal was associated with disorders and found them to be inconclusive.

No evidence was found that would prove a link between thimerosal-containing vaccines and autism, attention deficit-hyperactivity disorder, speech or language delays or other neurodevelopmental disorders, the committee said.

However, it said that as a precaution, the government should consider changing policies to reduce exposure to thimerosal as much as possible.

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